1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a novel self-centering mechanism for crash protection of interference microscope objectives.
2. Description of the Related Art
Interferometers used in surface profilometry utilize a microscope objective facing a sample surface mounted on a stage. These microscope objectives typically produce magnifications in the 1.5.times. to 100.times. range. Earlier designs operated at finite conjugates, in which case a real image is formed at a known, typically constant, distance from the objective. Most current microscope objectives operate at infinite conjugates, in which case a tube lens with a specific reference focal length (for example, 200 mm for Nikon and Leica, 180 mm for Olympus, and 164.5 mm for Zeiss objectives) is used in conjunction with the objective to form a real image. Infinite conjugate objectives have a focal length that varies inversely with the magnification (power) according to the relation f.sub.objective =f.sub.Reference /power. For example, a 50.times. Nikon objective has a focal length of 4 mm; a 20.times. Nikon objective of 10 mm; and so on. Very high magnification objectives have correspondingly short focal lengths, as well as very short working distances. The working distance of a microscope objective (or other lens) is the distance from the objective to the mechanical surface closest to the objective. A typical working distance for a 100.times.-magnification achromatic objective is approximately 0.3 mm.
As a result of such short working distances, the process of focusing with high magnification objectives always carries the possibility that the objective may contact the sample (a very undesirable event referred to as "crashing" in the art). In order to protect the objective's optical components from damage when a crash occurs, manufacturers typically incorporate the objective with a spring-loaded mechanism that allows the axial motion of the objective away from the sample surface when upward pressure is applied upon contact. As illustrated in FIGS. 1A and 1B, the operational components of such conventional mechanisms consist of two cylindrical surfaces providing a telescopic coupling between the objective and its housing. The objective 10 includes a cylindrical inner sleeve 12 that is adapted for relative axial motion within the conforming inner surface of the housing 14 of the microscope (which thus functions as an outer sleeve for the inner sleeve 12). The microscope objective 10 is urged forward by a spring 16 along the optical axis of the objective (i.e., its longitudinal axis). Thus, the objective is free to move inward when pushed as a result of an impact with a sample and is able to return to its original forward position upon release. A travel slot 18 and a pin 20 are used to prevent the objective 10 from rotating relative to the housing for the assembly.
The cylindrical connection between the inner sleeve and the housing of the crash protection mechanism necessarily includes a gap between the abutting surfaces of the two parts which may allow the position of the objective to shift in a radial direction. This gap, due in part to tolerances of manufacture and in part to the clearance necessary to permit the substantially frictionless axial translation of the objective, may produce misalignments whenever a crash occurs even though the objective is returned to its original axial position. Such radial shifts are often sufficient to cause measurement errors or require recalibration of other portions of the objective, especially in Linnik-interferometric applications.
Accordingly, there is still a need for a mechanism that protects a high-power interference objective from crash damage and that, in addition, prevents the radial shifting of the objective upon release from contact with the sample surface. This invention is directed at providing a novel approach to that end.